This invention relates to a method for starting the operation of a resistance furnace or melter. More specifically, this invention relates to a method for starting the operation of a resistance furnace for melting wherein the heating occurs by the resistance of the material to be melted to the passage of current therethrough, and the material is essentially electrically nonconductive when in a solid state.
The production of material such as glass which has a high melting temperature, requires large quantities of fossil fuels, such as natural gas, oil or producer gas made by treating coal. Due to increased costs and potential shortages of these fuels, attention is being given to alternative heating methods. Ceramic lined all electric glass melters are gaining broader use in the glass industry because of high capacity, long life, low volatilization and superior glass quality in addition to saving of fossil fuels and lessened environmental pollution.
In this type of melter the electrical current is passed between immersed electrodes through the molten glass, where the electrical energy is converted to heat by an amount equal to the current squared times the resistance of the molten glass (I.sup.2 R), called the Joule effect. Dissipation of energy directly into the glass allows the melter to thoroughly melt even the most difficult glasses and insures high glass quality. Glasses with high iron or chrome content are sometimes difficult to melt by external heating means because infrared absorption by the iron and chrome reduces the effective thermal conductivity of the glass at high temperature. By using a resistance melter, glasses with a high iron content can be melted without difficulty.
One obstacle with the use of a furnace or melter of this type is the difficulty in establishing operation of the melter when used with materials such as glass which are essentially electrically nonconductive when in a solid state, but which become more electrically conductive when in a molten state. Since most glasses do not become adequately conductive until heated to a temperature in excess of 700.degree. C. a good deal of heating is necessary before the melter itself becomes operational.
At present, such material is generally melted by heating with fossil fuels and using the hot gases to melt the material. This requires that a refractory cover or crown be installed over the melting cavity, even though it is used only during start-up. A gas or oil start-up also requires an off-gas treatment system during start-up to meet governmental air quality standards. Another start-up method might be to use radiant heating above the melting cavity, however, some glass compositions contain large amounts of iron oxide which insulates the material under the surface from heat. This is a particular problem when attempting to start a melter containing glass which has frozen in place and in which the electrodes are submerged far below the surface of the glass.